1. Field of the Invention
The invention relates to the novel synthesis of novel substituted derivatives of tetrathiafulvalene, tetraselenafulvalene and dithiadiselenafulvalene and their conducting charge transfer salts with tetracyano-p-quinodimethane.
2. Prior Art
Considerable interest has been found recently in the study of highly conducting organic charge transfer salts. Most attractive of these systems are the tetracyano-p-quinodimethane (TCNQ) salts of tetrathiafulvalene (TTF), tetraselenafulvalene (TSeF) and dithiadiselenafulvalene (DTDSeF). These salts display exceptional electrical conductivity and metallic behavior over a wide temperature range.
Presently, interest has been focused on substituted derivatives of the tetraheterofulvalenes, where hetero means S and/or Se which are hereafter referred to as fulvalenes. Substituted fulvalenes are of interest because they alter the conductivity of their charge transfer salts.
Prior attempts to synthesize unsymmetrically substituted fulvalenes have been by a cross coupling reaction described by M. Narita and C. Pittman, Jr. Synthesis, 1976, 495. The reaction can be generalized as follows: ##STR2## where R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are selected from various organic groups such as alkyls, aryls, esters, sulfur containing compounds and the like. This method of synthesis, however, has its obvious drawbacks, in that there are three possible products, which are most difficult to separate. For cross coupling reactions multi-step procedures are sometimes required to obtain just one of the components to be coupled. The variety of substituents that can be substituted is severely limited to reagents that will not react with the coupling reagents. Narita et al further discloses tetra carboxy substituted tetrathiafulvalenes from which the dicarboxy substituted heterothiafulvalenes are formed by decarboxylation. Notably, the reference does not teach or suggest a mono carboxy substituted tetrathiafulvalene. This is probably due to the fact that the decarboxylation reaction for the mono carboxy reaction is so much faster than for the dicarboxy reaction, that it, the mono carboxy product is not formed or at best only transitory, thus never isolated. Attempted substitution of TTF by direct action of a reagent such as a halogen has been shown to yield not the substituted derivative, but rather the unsubstituted radical cation salt. (F. Wudl et al, JCS Chem. Commun. 1970, 1453). This points up the difference between the chemistry of TTF and other sulfur heterocycles such as thiophene TTF. Thiophene is related to TTF in that they both have the C--S--C.dbd.C linkage in common. However, while direct substitution can be made to thiophene by a number of methods (F. F. Blicke, Heterocyclic Compounds, John Wiley & Sons, N. Y. pg. 208), it appears that the only substitution common to both TTF and thiophene is that of lithiation and subsequent reaction thereof.
In summary; prior art attempts to prepare unsymmetrically substituted TTF or TSeF derivatives have used mixed coupling reactions that give multi-product mixtures that are difficult to separate and which yield only a limited variety of substituents. Direct substitution methods have resulted in oxidative attack on the central double bond and have yielded only radical cation salts.
It has been discovered here that substituted fulvalene compounds can be synthesized through a metal organic fulvalene lithium compound. It is likely that sodium or other alkali metals would function as well. The synthesis using the fulvalene lithium can be reacted in most cases in one step to form the desired substituted fulvalene compound. Additional derivatives can then be obtained if desired via further reactions. The present method has the advantages over the cross coupling method in that it is shorter, more direct, versatile and requires fewer starting materials.